The present invention relates to methods of screening for compounds for use in treating and modulating an inflammatory response and, more particularly, to such methods of screening for compounds which mimic the effects of galectin-1.
Inflammation is the reaction of vascularized tissue to local injury. This injury can have a variety of causes, including infections and direct physical injury. Upon injury, the clotting system and plasmin systems are initiated together with the appropriate nervous system response to generate an initial response to facilitate immune activation. Increased blood flow, capillary permeability and chemotactic factors, including those of the complement cascade, modulate neutrophil migration to the damaged site. Neutrophils are the predominant cell type involved in acute inflammation, whereas lymphocytes and macrophages are more prevalent in chronic inflammation. The inflammatory response can be considered beneficial, since without it, infections would go unchecked, wounds would never heal, and tissues and organs could be permanently damaged and death may ensue. However, inflammation can also be potentially harmful. Inflammation causes the pathologies associated with rheumatoid arthritis, myocardial infarction, ischemic reperfusion injury, hypersensitivity reactions, and some types of fatal renal disease. The widespread problem of inflammatory diseases has fostered the development of many "anti-inflammatory" drugs. The ideal drug would be one that enhances the positive effects resulting from the inflammatory response, while preventing the potentially harmful side-effects of the inflammatory response.
The inflammatory response in regard to blood cells includes adhesion of circulating neutrophils, the most abundant phagocytic cell in the blood, to activated endothelial cells that form the lining of blood vessel walls. The adherent neutrophils are subsequently activated and emigrate from the blood into the surrounding tissue in a process termed diapedesis. The activated cells then begin engulfing microorganisms in a process termed phagocytosis. During this process the activated neutrophils release a variety of degradative enzymes, including proteolytic and oxidative enzymes into the surrounding extracellular environment. The mechanisms by which neutrophils adhere, become activated, and emigrate from the blood are currently major topics of research around the world.
Some of the important factors that mediate endothelial-neutrophil adhesion to initiate the inflammatory response are: bacterial products (e.g., endotoxin), complement fragments (e.g., C5a), chemotactic peptides, leukotriene B.sub.4 (LTB.sub.4), platelet activating factor (PAF), transferrin, and cytokines (e.g., IL-1 and TNF). These mediators stimulate activation of neutrophils and/or endothelial cells leading to expression of important adhesion molecules, such as integrins and selectins that mediate neutrophil binding. The adhesion of neutrophil to activated endothelium leads to neutrophil activation. Activated neutrophils have enhanced adhesion properties and the cells are highly migratory. The cells are also chemotactic and phagocytic. It is largely through their phagocytic activity that neutrophils promote clearance of infectious organisms.
Causes of inflammation are generally categorized as either infectious or non-infectious. Infectious diseases involving bacteria and viruses and other parasites, are usually treated by drugs that directly attack the infectious organism, (e.g. antibiotics such as penicillin and sulfonamides).
Non-infectious diseases in which neutrophils play a role in tissue damage include gout, rheumatoid arthritis, immune vasculitis, neutrophil dermatoses, glomerulonephritis, inflammatory bowel disease, myocardial infarction, ARDS (adult respiratory distress syndrome), asthma, emphysema and malignant neoplasms. Millions of people each year in the U.S. are treated for the above conditions. There are currently only a limited number of treatments for these chronic diseases which are usually characterized by long term morbidity and disability.
There can also be negative aspects to cytokine-mediated cell activation. The longer that activated neutrophils survive, the longer they continue to release enzymes and inflammatory mediators that can cause potentially harmful side effects. Though the half-life of circulating neutrophils is 6-8 hours, the extravascular survival of the activated cells can approach four days. Since it is well established that the numbers of activated neutrophils and their degree of activation is directly related to tissue injury, a tremendous amount of research has been done to identify compounds that inhibit the neutrophil response or that can decrease the number of activated neutrophils.
In vivo, as neutrophils die, they are recognized and phagocytosed by tissue macrophages, a process which is critical for resolution of the inflammatory response. In vitro, neutrophils undergo spontaneous apoptosis over a period of several days, which can be either enhanced or inhibited by cytokines and other mediators. Interestingly, phagocytosis of neutrophils was recognized by Metchnikoff over a hundred years ago (E. Metchnikoff, Lecture VII. In Lectures on the comparative pathology of inflammation. (F. A. Starling, ElH. Starling, eds.),; Kegan, Paul, Trench and Trubner [translated from French], 107-131, 1893). Phagocytosis of dying neutrophils is now recognized as the prime mode of resolving inflammation (J. Savill, J. Leukoc. Biol., 61:375, 1997).
There are a number of factors that have been shown in vitro to slow the apoptosis of neutrophils. These include lipopolysaccharides derived from bacteria and lineage-specific cytokines such as granulocyte-macrophage colony-stimulating factor and glucocorticoids. The inhibition of apoptosis by these agents may be advantageous in that the neutrophils are allowed to live longer and scavenge microorganisms. This also allows time for monocytes to differentiate and become effective against the infectious organisms. Conversely, neutrophil apoptosis is known to be accelerated by ligation of the cell surface death receptor Fas to the Fas ligand (K. Iwai, T. Miyawaki, T. Takizawa, A. Koono, K. Ohta, A. Yachie et al., Blood, 84:1201, 1994; W. C. Liles, P. A. Kiener, J. A. Ledbetter, A. Aruffo, S. J. Klebanoff, J. Exp. Med., 184:429, 1996), phagocytosis of bacteria (R. W. G. Watson, H. P. Redmond, J. H. Wang, C. Condron, D. Bouchier-Hayes, Escherichia coli. Journal of Immunology, 156:3986, 1996), and the binding of targets for the neutrophil integrin Mac-1 (A. Coxon, F. J. Barkalow, S. Askari, P. Rieu, A. H. Sharpe, U. Von-Andrian et al., J. Leukoc. Biol. (Suppl.) 64, 1996).
Work prior to the present invention has been directed toward identification of compounds that can control or regulate the inflammatory response. There are both chemically-derived drugs and bioactive proteins, termed cytokines, which have activity in regard to the inflammatory response. Chemical drugs used to treat inflammation are divided into two major classes, the steroidal and non-steroidal anti-inflammatory drugs. Steroidal anti-inflammatory drugs include the corticorsteroids, such as prednisone, methylprednisolone and cortisol. Non-steroidal anti-inflammatory drugs, the so-called NSAIDs, include aspirin, ibuprofen, indomethacin and diflunisal. In addition, tissues and blood cells release a variety of bioactive proteins, termed cytokines, that can have either anti- or pro-inflammatory activity. The pro-inflammatory mediators include IL-1, IL-8 and TNF-.alpha. and the anti-inflammatory proteins include IL-10. Another protein that indirectly has pro-inflammatory activity, and which is commercially produced by the biotechnology industry, is granulocyte-monocyte colony stimulating factor (GM-CSF). GM-CSF causes proliferation of neutrophils in the bone marrow and is used to treat patients undergoing chemotherapy who suffer from neutropenia (low neutrophil blood counts). A discussion of all these anti- and pro-inflammatory compounds can be found in Internal Medicine, Third Edition, (1990) by J. S. Stein, Editor-in-Chief, pp. 945-1239, Little Brown, Boston; and Robbins Pathologic Basis of Disease, Fourth Edition (1989) by R. S. Cotran, V. Dumar and S. L. Robbins, pp. 39-86, W. B. Saunders Co., Philadelphia.
In addition, as noted above, it is now known that the neutrophil adhesion to activated endothelium is a prerequisite for the inflammatory response. Proteins expressed by activated endothelium which are critical for neutrophil adhesion are selecting, such as P-selectin and E-selectin, and the immunoglobulin (Ig) superfamily members, such as CD54 (intercellular adhesion molecule-1 or ICAM-1). Neutrophils also express surface adhesion molecules, such as the .beta.2 integrin LFA-1 (CD11a,b,c/CD18), which binds to ICAM-1, and L-selectin, which binds P-Selectin glycoprotein ligand-1 (PSGL-1) on already adherent neutrophils and heparan sulfate-related molecules on activated endothelial cells. However, of paramount importance to the initial steps in inflammation, is the adhesion of neutrophils to selecting on endothelial cells. The general roles of adhesion molecules in inflammation are discussed in "The Sensation and Regulation of Interactions within the Extracellular Environment: The Cell Biology of Lymphocyte Adhesion Receptors" (1990) by T. A. Springer, in the Annual Review of Cell Biology, Vol. 6:359-402.
Galectin-1 is just one of a family of related proteins, termed the galectin family of .beta.-galactoside-binding proteins (see "Galectins: A Family of Animal .beta.-Galactoside-Binding Lectins" (1994) by S. H. Barondes, V. Castronovo, D. N. W. Cooper, R. D. Cummings, K. Drickamer, et al., In Cell 76, 597-598). The known proteins are galectin-1 through -8. Galectins 1,2,4,6,7, and 8 are divalent. Galectins 1, 2 and 7 probably occur in a monomer.rarw..fwdarw.dimer equilibrium; however, only galectin-1 has been truly shown to undergo this equilibrium, ("Galectin-1, a .beta.-Galactoside-Binding Lectin in Chinese Hamster Ovary Cells: I Physical and Chemical Characterization" (1995) by M.-J Cho and R. D. Cummings in the Journal of Biological Chemistry 270, 5198-5206). Galectins 4, 6, and 8 are covalent dimers and can only exist as dimers. In contrast, galectins 3 and 5 exist as monomeric species. Galectin-3 has been proposed to have an effect of blocking apoptosis of certain cells ("Expression of Galectin-3 Modulates T-cell Growth and Apoptosis" by R. Y. Yang, D. K. Hsu, and F. T. Liu (1996) in the Proceedings of the National Academy of Sciences, United States of America 93, 6737-42), though this has not been demonstrated for human neutrophils.
Galectin-1 forms a homodimer of 14 kDa subunits and each subunit has a single carbohydrate-binding site. This lectin is unusual in that it is synthesized in the cytosol of mammalian cells where it accumulates in a monomeric form. The lectin is actively, but slowly secreted (t.sub.1/2.apprxeq.20 h), and the secreted form occurs as a "metastable intermediate" that requires glycoconjugate ligands to properly fold and acquire stability. The functional lectin exists in a monomer-dimer equilibrium with a K.sub.d of .about.7 .mu.M and the equilibrium rate is rather slow (t.sub.1/2.apprxeq. 10 h).
Several known cytokines have been proposed to have carbohydrate binding activity and galectins, such as galectin-1, may antagonize or promote their activities. Recently, it was shown that galectin-1 can cause death of T-lymphocytes ("Apoptosis of T Cells Mediated By Galectin-1" (1995) by N. L. Perillo, K. E. Pace, J. J. Seilhamer, and L. G. Baum, Nature 378, 736-9). T cells stimulated by antigen, but not resting T cells, were killed apoptotically by galectin-1. Perillo, et al., speculated that this apoptosis required the dimeric form of the lectin, but no direct evidence was presented for this idea. Furthermore, they did not define any specific changes in cell surface glycoconjugates accompanying T cell activation that might predispose T cells to die by apoptosis. Indeed, Perillo et al. stated that resting T cells bind galectin-1, but are not killed.
Galectin-1 has been used in therapeutic treatments for T cell-based autoimmune diseases in animal models; for example, experimental autoimmune encephalomyelitis (H. Offner, B. Celnik, T. S. Bringman, D. Casentini-Borocz, G. E. Nedwin, A. A. Vandenbark, Recombinant Human Beta-galactoside Binding Lectin Suppresses Clinical and Histological Signs of Experimental Autoimmune Encephalomyelitis, Journal of Neuroimmunology, 28(2):84, 1990), and experimental autoimmune myasthenia gravis (G. Levi, R. Tarrab-Hazdai, V. I. Teichberg, Prevention and Therapy with Electrolectin of Experimental Autoimmune Myasthenia Gravis in Rabbits, European Journal of Immunology, 13(6):7, 1983).
Galectin-1 can inhibit the growth of certain types of cells ("Identification of an Autocrine Negative Growth Factor: Mouse Beta-Galactoside-Binding Protein is a Cytostatic Factor and Cell Growth Regulator" (1991) V. Wells and L. Mallucci, Cell 64, 91-7). However, the specific findings regarding the effects of galectin-1 on activated neutrophils demonstrated herein are unanticipated by previous studies. Indeed, as noted above, resting T-cells bind galectin-1 but are not killed by this binding.
As indicated above, acute inflammatory reactions are often initiated by invasive organisms and injury; however, there are many other disease states characterized by acute inflammation.
It is with this in mind that physicians attempt to control the acute inflammatory reaction with glucocorticoids, non-steroidal anti-inflammatory agents and cytotoxic drugs. These general immunosuppressive agents are helpful but are not specifically directed to the resolution of the acute inflammatory response. Agents specifically directed to the control of the acute reaction would be less likely to promote the systemic side effects produced by general immunosuppressives and would permit better control over a potentially life-threatening reaction. Effective inhibitors of causative factors of the acute inflammation which have fewer side effects would therefore be most useful.